Fuel assemblies for nuclear reactors are generally provided in the form of fuel element or rod arrays maintained by a structure which includes a plurality of welded spacer grids, a lower end fitting and an upper end fitting. Guide tubes provide the structural integrity between the lower end fitting, the upper end fitting and the spacer grids intermediate the ends of the fuel assembly. The spacer grids define an array of fuel rods which, typically, may be rows and columns of up to 20 rods each. One such spacer and support grid is disclosed in U.S. Pat. No. 3,481,832. See also U.S. Pat. No. 4,803,043.
The typical fuel element support grid for supporting a spaced array of nuclear fuel elements or rods intermediate their ends includes a generally quadrangular or other polygonal perimeter. A plurality of fuel element compartments or cells within the perimeter are defined by first and second grid-forming members or strips welded to the perimeter and joined to each other at their lines of intersection. The grid-forming members of the fuel element support grid are slotted for part of their width along lines of intersection with the other grid-forming members of the array such that they may be assembled and interlocked at their lines of intersection in what is termed "egg-crate" fashion. The grid-forming members of one embodiment of the present invention are also bent at points intermediate the walls of the compartments for reasons discussed in U.S. Pat. No. 3,423,287. The wavy-strip structure of this embodiment provides a good strength-to-weight ratio without severely affecting the flow of cooling or moderating fluid through the grid of the nuclear reactor. The grid strip bends typically act as integral arches and act with integral projecting springs for engaging and supporting the fuel elements within the compartments. Thus, at each fuel rod grid position in the fuel assembly, axial, lateral and rotational restraint is provided against fuel rod motion due to coolant flow, seismic disturbance or external impact. The spacer grids also act as lateral guides during insertion and withdrawal of the fuel assembly from the reactor. Since separate arches out of the plane of the grid-forming members are not necessary, a minimum pressure drop is accomplished.
In U.S. Pat. No. 3,764,470, a flow twister, mixing vane, or fluid flow directing vane was disclosed for redirecting the cooling fluid in the channels between the spaced parallel nuclear fuel elements. Those twisters were U-shaped metal sheets which straddled one grid member at an intersection with the free ends of the "U" folded on themselves to form two pairs of oppositely directed spirals and a pair of slots receiving the other grid member. The purpose of the twisters was to direct cooling fluid inwardly toward and spirally around the adjacent fuel rods. The desirability and theory of their use is described in the "Background of the Invention" of U.S. Pat. No. 3,764,470. The same background is applicable to the invention described herein. This patent also shows bent or "wavy" grid-forming members which define integral arches.
As provided in U.S. Ser. No. 048,336, fluid flow directing vanes or "mixing vanes" were integral to the strips and provided improved strength for the grid and improved hydraulic performance of the type previously provided by the separate "twisters" of U.S. Pat. No. 3,764,470. It was noted that a major advantage of the fluid flow directing vanes being integral was the reduced chance of them becoming loose parts or debris within the flow stream circulating in the reactor in a manner which could damage the internals of the reactor.